The field is veneer drying and handling. More particularly, the inventive subject matter relates to a contact drying process for veneer and several material handling devices.
In the manufacture of plywood panels, dried veneer sheets are utilized. The veneer sheets are peeled from a log and then clipped to size before being dried to a predetermined moisture content suitable for adhesive bonding. The desirable dry veneer characteristics for adhesive application and manufacture into plywood are: a moisture content that is uniform and consistently within a predetermined narrow range; the veneer should have smooth surfaces for adhesive application and material handling; the surfaces of the dry veneer should be free from extractives and residues; the veneer sheets should be dimensionally stable; and the veneer sheets should have a uniform thickness.
Conventional veneer drying systems presently used in plywood or veneer manufacturing plants are typically long continuous hot-air dryers. The wet veneer sheets, having a green moisture content of anywhere from 30 to 250%, are conveyed longitudinally through a hot-air dryer. Hot air impinges upon the surface of the veneer as it passes through the dryer, transferring heat to the veneer sheet thereby drying the veneer and generally reducing its moisture content to a desired range of from 0-10% for softwoods and from 5-20% for hardwoods.
Owing to the variability of the moisture content of incoming wet veneer, thermal stresses, and resulting shrinkage that occurs during the typical drying process, many of the above-mentioned desirable characteristics are not consistently obtained. For example, conventionally dried veneer usually has a range of moisture contents within the resulting dried veneer and a certain percentage of veneer that passes through the dryer must be redried in order to reduce the moisture content to the finally desired level. In addition, certain areas within a sheet of veneer are susceptible to over-drying, which also is undesirable in that in the over-dried areas, that is bone dry, additional adhesive is required to result in good adhesive bonding. Typically, the veneer that exits from a conventional dryer is oftentimes wavy since it is unrestrained during drying and, therefore, presents material handling problems which can result in additional wood losses subsequently in the plywood manufacturing process. Typically, shrinkage in the veneer will occur in the tangential direction, that is tangential to the annual rings of the log which is on the order of from 5 to 10% of the initial veneer dimensions. Of course, it will be appreciated by those skilled in the art that if this loss could be reduced, then the typical first clipped size could be reduced, thereby conserving valuable wood.
Additional wood loss occurs in conventional drying processes due to the nonuniform composition of conventionally dried veneer. If there is a great deal of redrying that must be done due to nonuniform drying, then typical losses during the redrying process can be on the order of from 3 to 8% of veneer primarily due to its material handling losses. Additionally, it has been determined that further processing damage can result in an additional 3 to 7% wood loss from such areas as binding in the adhesive spreaders, uneven edges catching in various downstream mechanical processes, and the like.
Not only is there a great deal of wood loss within conventional drying processes but there is also the inherent problem stemming from the nonuniform drying characteristics due to the use of impinging hot air on unrestrained veneer. This results primarily from the nonuniform drying rates within the wood itself due to its initial nonuniform moisture content. This results in quantities of veneer exiting a veneer dryer being both overdried and underdried, which in turn results in redrying with its losses, other downstream problems such as "blows" and gluing problems stemming from "dryout" and "undercure." Dryout is that condition where the veneer is bone dry and, among other problems, allows the adhesive to penetrate the veneer more than desirable and during pressing of the plywood panels, the adhesive will not bond properly. Undercure is that condition where inadequate adhesive penetration occurs at the surface of the veneer and, consequently, adhesive requires a longer time within the panel press to cure. Blows are most severe and result where underdried wet veneer is placed in a hot press and excessive vapor is created under pressure causing separation or delamination of the plies. Of course, these typical causes of delamination are well known to those skilled in the art and are known to be caused by the nonuniform moisture contents within the conventionally dried veneers. The most optimum moisture content range in a sheet of dried veneer is typically from 1 to 5% and preferably the content will be substantially uniform within the range. By having a uniform, narrow range, adhesive penetration into the surface of the dried veneer is also uniform and then, when the laid-up plies are pressed into the panel, uniform curing of the adhesive will occur. As previously mentioned, however, such a uniform moisture content within a narrow range is nearly impossible in conventional veneer drying processes due to the substantial variability in initial moisture content within the green veneer sheets. In some veneer the moisture content may range from 30 to 150% within the same sheet. Such sheets may be comprised in part of heartwood and in part of sapwood, thereby causing the wide variation.
With existing conventional dryers and redryers, a substantial amount of energy is required per unit of dried veneer and in terms of steam usage, a typical range would be from 2,100 to 3,000 pounds of steam per hour per 1,000 square feet of dried veneer. Electrical energy requirements may vary typically from 520 to 1,500 hp. Of course, it will be recognized that, with today's high energy costs, these energy requirements represent a significant cost element in drying veneer. In addition, with the typical 10 to 30% redry rate, an additional amount of energy is required. A significant amount of the energy going into the drying system is wasted in that the heat transfer characteristics of impinging hot air onto veneer sheets is extremely inefficient, and thus a significant amount of energy is required to reduce the moisture content to acceptable levels. As the thickness of the veneer to be dried increases, as well as the average initial moisture content, even greater amounts of energy are required per unit of dried veneer to achieve the desired final moisture content. Of course, it is well known that within existing commercially available veneer dryers, the veneer which is thick and has a high moisture content is left in the dryer for a longer time, simply by reducing the speed with which it travels through the dryer. As pointed out above, as the hot air impinges on the veneer for longer time periods, the quantity of overdried veneer increases.
It is also a recognized problem that with conventional dryers the traveling veneer, usually supported on wire cables or rollers has a tendency to catch on internal parts causing machine stoppages requiring an operator to physically open the dryer and remove the accumulation of veneer. This results in a loss of valuable veneer in addition to slowing down the dryer line and causing other problems. This is, in part, caused by the severe strains producing waviness due to the high temperatures within the dryer. Additionally, if veneer sheets are allowed to remain stationary within the dryer, they can get so hot that ignition can occur causing fires which, of course, represent a safety hazard to persons and property. These just-mentioned operating problems are inherent in most conventional veneer dryers and are dealt with simply by expending the cost to solve the problem when it arises.
Another aspect of conventional veneer drying is the reduced capacity of the primary dryer caused by a requirement to redry 10 to 30% of the veneer. Redrying takes the capacity of the primary dryer to accommodate the time for redrying the partially dried veneer.
Yet a further problem with conventional veneer dryers, one that is of increasing importance, is the emmission of so-called "blue haze." Blue haze is essentially a vapor containing particulate matter that is generated from the drying process consisting in part of hydrocarbons in particulate form. Most conventional dryers that merely exhaust their vapors into the atmosphere without treatment do not meet minimum environmental standards now required in most states. There are various control means for reducing the blue haze, but they are quite expensive and represent an added capital cost to the veneer drying process and, in addition, the collected residuals must be disposed of, representing yet another additional cost. It would thus be desirable to have a veneer drying process that reduces or eliminates the generation of the blue haze vapor.
The process for drying veneer employed in the present invention is generally known; however, it has never been developed as a primary drying process. The general process is drying veneer through the use of heated metal plates that are positioned in intimate contact with the wet veneer sheets with a pressure then applied to the veneer while drying takes place to the final uniform moisture content. Past research efforts in contact drying have resulted in published data on the subject, but there is nothing in the prior art that discloses a commercially usable contact drying system that consistently provides uniformly dried veneer and that reduces or eliminates the abovementioned problems usually present in today's commercially available impinging hot-air dryers.
It is known to use conventional hot presses with flat heated plates for the purpose of flattening and redrying hardwood veneers prior to their being utilized in hardwood plywood manufacture. These hot presses are of a conventional design and simply accept a number of thin sheets of hardwood veneer for the application of heat and pressure for a time period to flatten and redry the already substantially dried hardwood veneer. Such a typical hot press would not be commercially usable for the primary drying of substantial quantities of green veneer or partially dried veneer for use in commodity-type plywood, nor would such a press be useful for drying single sheets of high-value, thin hardwood veneer.
As noted above, the veneer produced by a commercial dryer should have a uniform moisture content, be substantially flat, have a uniform thickness, and be a generally high-quality veneer with a minimum of splits or other surface blemishes. In developing a commercially usable contact drying system for veneer, it will be recognized that, in addition to providing the desired results with respect to the dried veneer, the machinery system must be capable of economic, reliable, and safe operation. One of the requirements is to feed the wet veneer or partially dried veneer sheets into the contact drying system uniformly, quickly, and without damaging the sheets. A further need of the machine system is that the veneer sheets be properly positioned between the heated metal plates of the drying apparatus and, therefore, it is essential that some suitable aligning means be provided. Yet another need within the machine system is for an unloading apparatus to quickly unload the dried veneer from the contact drying system. Ideally, an unloading apparatus would singulate the dried veneer sheets for downstream processing such as cooling or stacking.
With respect to a commercially usable contact drying system and its fit within the overall plywood manufacturing process the drying system must be compatible with production from a veneer lathe in terms of its capacity and, likewise, must also produce dried veneer at a rate to make the downstream panel manufacture economic. Thus the contact drying system must have the flexibility of structure, design, and operating parameters to meet both desired production goals and veneer quality requirements.
In the contact redrying system, any redryer must be able to redry the amount of partially dried veneer that results from the primary drying process. In the situation where a typical known commercial dryer is the upstream process, the amount of veneer to be redried will be substantially greater than if the primary drying process is a contact drying system.
When veneer is generated at a lathe, it flows in a continuous ribbon and is clipped to a predetermined size and then graded and stacked for downstream processing. The stacks of veneer that result must be acceptable for handling within a contact drying system and thus the loading station must be capable of handling the heavy stacks of wet veneer that can weigh on the order of 6,000 pounds. Similarly, the dried veneer flowing out of the contact drying system is again graded and stacked and the outfeed system must be compatible with this downstream requirement.
Accordingly, from the foregoing, it is an object of the invention to dry wet or partially dried veneer to a uniform predetermined moisture content. It is a further object to produce dried veneer that has flat, smooth surfaces affording easier downstream handling. Another object is to provide dried veneer having a uniform thickness. Yet another object of this invention is to reduce the shrinkage during drying, thereby increasing yield. A further object is to reduce splits and other downgrading characteristics. Still another object is to dry the veneer to a uniform moisture content with less energy than with conventional veneer dryers. Another object is to reduce the blue haze emissions usually resulting from veneer drying. Yet a further object of the invention in the redrying system is to quickly redry partially dried veneer without producing overdried veneer or excessive shrinkage. It is likewise an object of this invention to provide contact drying systems that are economic with respect to the initial capital costs and which, in the economic sense, provide substantial benefits over conventional veneer drying systems. Yet another object is to provide contact drying systems that are compatible with the plywood manufacturing system from the veneer lathe to the panel press.
These and many other objects of this invention will be more completely understood and appreciated after reading the detailed description to follow while referring to the attached drawings.